ANCIENT LIFE ON THE EAETH 33 



erroneous impression, due to the very unequal 

 lengths of time represented by the different periods. 

 Making every allowance for the possibility that the 

 rates of denudation and deposition may have been 

 greater in past times than now, still we must admit 

 that the relative thickness of the sedimentary rocks 

 of each period is a rough measure of the relative 

 length of time it represents ; and I suppose that 

 every geologist will agree that the Huronian, the 

 Algonkian, the Cambrian, and the Ordovician were 

 collectively at least equal in duration to all the 

 periods that came after them that is, they repre- 

 sent at least one-half of the time since life first 

 appeared on the earth. But certainly the changes 

 which have taken place in animals, and especially in 

 plants, since the commencement of the Silurian 

 period, are far greater than those that went before, 

 both in the addition of new groups and in the extinc- 

 tion of old ones ; so that the rate of variation must 

 have increased and not diminished with time. It 

 was this Slow rate of variation in ancient times that 

 enabled the early Palaeozoic genera to spread so 

 much more widely over the earth than do the genera 

 of the present day. 



Extinction of Groups. The diminution or decay 

 of a whole group of animals first began with the 

 Graptolites in the Upper Ordovician, and they finally 

 became extinct in the Carboniferous. The same 

 process commenced with the Trilobites in the 

 Silurian period, and they became extinct in the 

 Permian. Can we trace any cause for this gradual 

 process of decline in numbers? The existence, in 



